1. Field of the Invention
The present disclosure provides an apparatus and method for testing the structural integrity of tubing and casings used in a borehole. In particular, the present disclosure discusses an apparatus and method using ultrasonic waves to estimate the stress on tubulars in a borehole environment.
2. Description of the Related Art
The environmental conditions encountered by production casing and tubing used in hydrocarbon recovery can result in stress buildup in the tubing. This stress in the tubing may come from pressure and temperature variations during production, movement of the formation due to pressure depletion, “flow” of salt formations, etc. This stress may eventually lead to casing or tubing collapse or shear, rendering the well inoperable. Prior art methods have generally involved waiting for the buildup of this stress to a point where mechanical deformation occurs before the stress can be detected.
Stress buildup may also occur in a drillstring during the drilling of a borehole. During drilling operations, it is not uncommon for the drillstring to get stuck. To recover the stuck pipe, it is first required to determine the upper most ‘free’ point of the drillpipe. This is done by measuring the torque and/or pull induced from the surface or the physical stretching of the drillpipe due to this torque or pull.
Stress in a casing or tubing may be in the form of an axial load, circumferential torque, or a bending moment. Although stresses are applied on the drilling equipment while in use in the borehole environment, testing for wear typically occurs uphole or in a laboratory, often by observing the residual stress on the mandrel from its use. In general, when a stress is applied to a material and then removed, a residual stress remains on the material. This residual stress is often observed by checking for atomic dislocations at the crystalline level of the material and can be used to determine properties related to the structural integrity of the material. Various methods have been designed to observe residual stress on materials, including X-ray diffraction techniques, determining magnetic permeability, and ultrasonic testing.
Changes in ultrasonic wave propagation speed, along with energy losses from interactions with materials microstructures are often used to nondestructively gain information about properties of the material. An ultrasonic wave may be created in a material sample, such as a solid beam, by creating an impulse at one region of the sample. As the wave propagates through the sample, stresses and other material changes or defects affect the wave. Once the affected wave is recorded, the nature of the stresses of the material can be determined. Measurements of sound velocity and ultrasonic wave attenuation can be related to the elastic properties that can be used to characterize the texture of polycrystalline metals.
Velocity measurements are of interest in longitudinal waves propagating in gases, liquids, and solids. In solids, transverse (shear) waves are also of interest. The velocity of a longitudinal wave is independent of a sample's geometry when the dimensions at right angles to the sample are large compared to the sample area and to the wavelength. The velocity of a transverse wave is affected little by the physical dimensions of the sample. The relationship between stress and velocity has been discussed for example by Cantrell and Chern, “Relative Slope Invariance of Velocity-Stress and Strain-Stress Curves,” Ultrasonics Symposium, 1981.
Measurement of ultrasonic velocity is performed by measuring the time it takes for a pulse of ultrasound to travel from one transducer to another (pitch-catch scenario) or return to the same transducer (pulse-echo scenario). Another measurement method compares the phase of the detected sound wave with that of a reference signal, wherein slight changes in the transducer separation are seen as slight phase changes, from which the sound velocity can be calculated. These methods are suitable for estimating acoustic velocity to about 1 part in 100. Standard practice for measuring velocity in materials is detailed in American Society for Testing and Materials (ASTM) Publication E494. Residual stress measurements in cylinders have been discussed for example by Frankel et al., “Residual Stress Measurement in Circular Steel Cylinders,” Ultrasonics Symposium, 1983.
An oriented measurement of magnetic permeability has also been used to determine stress. Several patents discuss the use of magnetic permeability to measure stress. U.S. Pat. No. 4,708,204 to Stroud discusses a system for determining the stuck point of pipe in a borehole including a wireline tool having an exciter coil and a receiver coil axially spaced from one another. The exciter coil is driven at a pre-selected low frequency and the voltage induced into the receiver coil is related to the magnetic permeability of a pipe through which the tool is run. A receiver coil voltage log is run of the section of pipe in the region of the stuck point first while that region is substantially free of mechanical stress. A second log of the same region is run with the pipe under mechanical stress. Comparison of the two logs determines the stuck point from the difference in magnetic permeability of the stressed pipe above the stuck point and the unstressed pipe below the stuck point.
European Patent Application EP 1 647 669 A1 discusses a method and apparatus for determining a stuck pipe. In one embodiment, a free point logging tool, having a freepoint sensor and, optionally, an acoustic sensor, is attached to a working line such as a wireline. The freepoint sensor acquires magnetic permeability data in a string of pipe, while the acoustic sensor acquires acoustic data in the pipe. Two sets of data for each sensor are acquired: one in which the pipe is unstressed, and one in which the pipe is stressed. The first set and second sets of magnetic permeability data are compared to determine the stuck point location of the pipe. The first and second sets of acoustic data are compared to determine the matter in which the pipe is stuck. EP 1 647 669 A1 references the use of travel time to measure stress but anticipates only a measure of axial travel time.
In petroleum exploration, time spent raising and lowering a drilling apparatus from and into a borehole is time that could otherwise be used in exploration and is thus costly. Historically, stress on a tubular containing drilling equipment used in a borehole has only been determined by looking for actual physical movement of the tubular (i.e., freepoint indicators) or by physical distortion of the tubular (i.e., casing inspection). Thus, it is desirable to perform stress testing of a drilling apparatus obtaining measurements downhole.